Magneto pinion slipping

[TimK]

Hello wiser heads

I'm having problems tightening the magneto pinion without disturbing the contact breaker. Service sheet 203 says "holding the contact breaker tighten the pinion nut". The problem I'm having is that I can't tighten the nut enough to grip the taper fully without disturbing the contact breaker.

I made up a tool tó lock the contact breaker assembly but just managed to strip the key on the contact breaker back plate (luckily without damaging the keyway on the magneto shaft). I was planning on changing this anyway as it's pretty knackered so it wasn't too much of a disaster.

The pinion nut is going fully home but isn't providing enough pressure to grip the taper - any ideas?

Cheers

Tim

[Klaus]

Hi Tim,
when you have timed the contakts in fully advance poisition as in the servissheet described, give the timing pinion a punsh with a sparkplug socket. This will bring the pinion on the taper without moving the contakts. Than you can tighten the nut.

cheers Klaus

[duTch]

 I had some grief fitting mine, as the pinion was butting hard up against the casing.  Originally I made a cone with some brass shim material which worked ok, but on the for-real practise  rebuild, I elected to dispense with the cone shim, and carve away the casing behind (about 0.025" from memory). It doesn't need a thousand tons of torque to hold it in place, so might be worth looking at ? 10K miles down da track is still rocketin' on ok

[TimK]

Thanks for your input Klaus and duTch, I've already tried tapping the pinion with a socket over the nut but it didn't seem to help, I think the next step may be making up some sort of small spacer as duTch suggests.

cheers

Tim

[orabanda]

Inspect the condition of the taper; there cannot be any burrs or high spots on it.
Richard

[groily]

Sounds as if the taper is pretty worn and/or there are mechanical clearance problems with the pinion/nut.

If  the nut simply bottoms on the thread before the taper is properly engaging, and if there is clearance between pinion and timing case with it tapped home, then might you be able to sort it with a thicker washer under the nut (or if it's an ATD, with a thicker horseshoe washer on the outboard end)?

In any event it's a bad idea to tighten a 3/8 BSF nut against the little 3BA thread and small brass key on the cb assembly - it's an unfair contest with only one winner!
A useful wheeze (I find, anyway) is to remove the earth brush from the mag casing and screw in a hard plastic or soft brass plug (5/16th BSF) until it contacts the armature. This should hold things exactly where you want them while you get the pinion into place and starting to grip.

If you don't have another decent contact breaker backplate, you might be able to reclaim the scrunched keyway with a home-made 'broach', or you could make it good with solder and Swiss files (painstaking but it will work). Or you could make another keyway exactly 180° opposite the damaged one. Its position is very important, but it is only a register and it shouldn't be doing any real work.

[Greybeard]

In some cases grinding the taper mating surfaces with grinding paste may help to get a nice match. I did this on my Austin Seven crankshaft.

[beezermacc]

I would be inclined to remove the magneto and investigate the problem on the bench. It's much easier to see what's happening. Also, check that a 3/8 nut will travel all the way down the thread before trying the ATD.

[TimK]

Thanks everyone, the magneto is now off the engine and investigations continue. The taper doesn't look too bad on either the magneto or the auto advance unit, but I think I'll lap it in with some fine paste just in case. I've checked the thread on the magneto spindle and it seems fine, the thread on the auto advance unit retaining nut seems fine as well.
I'll reassemble and try your idea groily of a threaded plastic plug to hold the magneto spindle in place until the auto advance unit grips on the taper. I'll let you know how I get on.

Cheers
Tim

[beezermacc]

When attaching the ATD to the mag it is important to keep the ATD components together. Keep the head of the nut, the C washer and cupped washer all trapped together as you turn the nut onto the magneto shaft. Some people make the mistake of pushing the ATD onto the shaft then tightening the nut which causes the left hand threads to bite so the nut feels tight before the ATD has gripped. The trick is to make sure the washers, nut and ATD are all held together, pull the ATD away from the bike to keep the parts trapped whilst tightening the nut. When the ATD is almost on the taper give it a push to grip it onto the taper then tighten the nut the last half turn or so.

[beezermacc]

Just remembered there's a handy(?) diagram on my ATD page......
https://sites.google.com/site/priorymagnetos/atd-units